We present a nanometer-scale correlation of the structural, optical, and chemical properties of InGaN/GaN core-shell microrods. The core-shell microrods have been fabricated by metal organic vapor phase epitaxy (MOVPE) on c-plane GaN/sapphire templates covered with a SiO2-mask. The MOVPE process results in a homogeneous selective area growth of n-doped GaN microrods out of the mask openings. Surrounding the n-GaN core, a nominal 5 nm thick GaN shell and 30 nm thick InGaN layer were deposited.
Highly spatially resolved cathodoluminescence (CL) directly performed in a scanning transmission electron microscope (STEM) was applied to analyze the selective Indium incorporation in the thick InGaN shell and the luminescence properties of the individual layers. Cross-sectional STEM analysis reveal a hexagonal geometry of the GaN-core with m-plane side-walls. Directly at the corners of the hexagon a-plane nano-facets with a length of 45 nm are formed. The overgrowth of the GaN core with InGaN leads to a selective formation of Indium-rich domains with triangular cross-section exactly at these nano-facets as evidenced by Z-contrast imaging. Probing the local luminescence properties, the most intense CL emission appears at the m-plane side-facets with 392 nm peak wavelength. As expected, the Indium-rich triangles emit a red-shifted luminescence around 500 nm.

We present low temperature cathodoluminescence (CL) characterization of non-polar GaN epitaxial lateral overgrowth (ELO) structures at various growth stages. The a-plane GaN ELO was grown on a-plane GaN template on r-plane sapphire by metal organic chemical vapor deposition (MOCVD). A 50 nm SiO2 mask with 4 µm mask / 6 µm window regions was used for selective growth aligned along the c-direction. Growth was promoted vertically out of the mask openings with a shift to lateral promoting growth by halving the V/III ratio of precursors. Finally, the structures were capped by an AlGaN layer.
The distinctly different growth domains of a-plane ELO GaN on stripe masks oriented along c-direction were directly visualized by highly spatially and spectrally resolved cathodoluminescence microscopy.
Distinct microscopic regions dominated by differing individual peak wavelengths originating from either basal plane stacking faults, prismatic stacking faults, impurity related donor-acceptor pair or (D0,X) emission as well as yellow luminescence are explicitly correlated to the different growth domains. A strong increase in luminescence intensity from the ELO wings in comparison to the coherently grown region is observed.
A 70 nm AlGaN film of 30% Al-concentration was deposited on a coalesced GaN ELO sample and hydride vapor phase epitaxy (HVPE) grown bulk GaN film by MOCVD. A comparison of the luminescence properties was made to probe the growth quality of the overgrown layer and AlGaN/GaN interface.
Acknowledgement: This work was supported by the National Science Foundation under Grant no. DMR-1309535.

We systematically studied the desorption induced GaN/AlN quantum dot formation using cathodoluminescence spectroscopy directly performed in a scanning transmission electron microscope (STEM). The GaN films were grown by metal organic vapor phase epitaxy (MOVPE) on top of an AlN/sapphire-template. After the deposition of a few monolayers GaN at 960°C a growth interruption (GRI) without ammonia supply was applied to allow for quantum dot formation. A sample series with GRI durations from 0 s to 60 s was prepared to analyze the temporal evolution systematically. Each quantum dot (QD) structure was capped with AlN grown at 1195°C.
Without GRI the cross-sectional STEM images of the reference sample reveal a continuous GaN layer with additional hexagonally-shaped truncated pyramids of 20 nm height and ~100 nm lateral diameter covering dislocation bundles. Spatially averaged spectra exhibit a broad emission band between 260 nm and 310 nm corresponding to the continuous GaN layer. The truncated pyramids exhibit only drastically reduced CL intensity in panchromatic images.
Growth interruption leads to desorption of GaN resulting in smaller islands without definite form located in close vicinity to threading dislocations. Now the emission band of the continuous GaN layer is shifted to shorter wavelengths indicating a reduction of GaN layer thickness. By applying 30 s GRI these islands exhibit quantum dot emission in the spectral range from 220 nm to 310 nm with ultra narrow line widths. For longer growth interruptions the QD ensemble luminescence is shifted to lower wavelengths accompanied by intensity reduction indicating a reduced QD density.

Growth of nonpolar and semi-polar GaN and GaN-based structures offers the opportunity to reduce quantum confined Stark effect and possibly increase indium incorporation, as compared to polar structures, for enhanced performance in green and longer wavelength light emitters. However, the development of the nonpolar and semi-polar GaN growth is hampered by the lack of suitable substrates. Silicon, despite its large thermal-expansion and lattice mismatch with GaN, provides the advantages of the availability of large-size wafers with high crystalline quality at low cost, good electrical conductivity, and feasibility of its removal through chemical etching for better light extraction and heat transfer. In this article, we overview the recent progress in epitaxial growth of nonpolar and semi-polar GaN-based structures on patterned Si substrates. Also discussed are structural and optical properties of the resulting material.

We present a nanometer-scale correlation of the structural, optical, and electronic properties of InGaN/GaN core-shell microrod LEDs: The microrods were fabricated by MOVPE on a GaN/sapphire template covered with an SiO2-mask. Through the mask openings, Si-doped n-GaN cores were grown with high SiH4 flow rate at the base. Subsequently, the SiH4 flow rate was reduced towards the microrod tip to maintain a high surface quality. The Si-doped GaN core was finally encased by an InGaN single quantum well (SQW) followed by an intrinsic GaN layer and a thick Mg-doped p-GaN shell.
Highly spatially resolved cathodoluminescence (CL) directly performed in a scanning transmission electron microscope (STEM) was applied to analyze the free-carrier concentration within the Si-doped GaN core and the luminescence properties of the individual functional layers. The CL was supported by Raman spectroscopy directly carried out at the same microrod on the thin TEM-lamella.
The cross-sectional CL of a single microrod resolves the emission of the single layers. CL and Raman measurements reveal a high free-carrier concentration of 7x1019 cm 3 in the bottom part and a decreasing doping level towards the tip of the microrod. Moreover, structural investigations exhibit that initial Si-doping of the core has a strong influence on the formation of extended defects in the overgrown shells. However, we observe the most intense emission coming from the InGaN QW on the non-polar side walls, which shows a strong red shift along the facet in growth direction due to an increased QW thickness accompanied by an increased indium concentration right at the intersection of generated defects and InGaN QW, a red shifted emission appears, which indicates indium clustering.

In the past few years, tremendous progress has been achieved on epitaxial growth and processing of group III nitride nano- and microrods. Furthermore, these growth improvements have allowed the fabrication of optoelectronic devices based nanorods as active elements, i.e. light emitting diodes (LEDs). However, their efficiency is still far behind the performance of conventional GaN-based light emitting diodes.
The controlled growth of GaN nanorods offers a potential benefit for achieving higher efficiencies of III-Nitride based optoelectronic devices due to a high surface to volume ratio. Nanorods have a very large active area compared to their footprint. Since the active region is wrapped around the three-dimensional core (for core shell structures), the active layer scales with the rod’s aspect ratio (i.e. the ratio of height and diameter). Therefore, by controlling their density, diameter and height, a tremendous increase of active surface can be achieved. Additionally, the low defect density in nanorods allows the characterization of single extended defects which is of high interest for a clear understanding of the formation of these defects.
In this study we present a direct nano-scale correlation of the optical properties with the actual real crystalline structure of single GaN nanorods using low temperature CL spectroscopy in a scanning transmission electron microscope (STEM). We concentrate on the crystalline quality, local In incorporation, n- and p-layer quality and defects of the complete structures.

Enhancement of optical and structural quality of semipolar (11‾22) GaN grown by metal-organic chemical vapor deposition on planar m-sapphire substrates was achieved by using an in-situ epitaxial lateral overgrowth (ELO) technique with nanoporous SiNx layers employed as masks. In order to optimize the procedure, the effect of SiNx deposition time was studied by steady-state photoluminescence (PL), and X-ray diffraction. The intensity of room temperature PL for the (11‾22) GaN layers grown under optimized conditions was about three times higher compared to those for the reference samples having the same thickness but no SiNx interlayers. This finding is attributed to the blockage of extended defect propagation toward the surface by the SiNx interlayers as evidenced from the suppression of emissions associated with basal-plane and prismatic stacking faults with regard to the intensity of donor bound excitons (D0X) in lowtemperature PL spectra. In agreement with the optical data, full width at half maximum values of (11‾22) X-ray rocking curves measured for two different in-plane rotational orientations of [1‾100] and [11‾23] reduced from 0.33º and 0.26º for the reference samples to 0.2º and 0.16º for the nano-ELO structures grown under optimized conditions, respectively.

Reduced electric field in semipolar (1122) GaN/InGaN heterostructures makes this orientation attractive for high efficiency light emitting diodes. In this work, we investigated indium incorporation in semipolar (1122) GaN grown by metal-organic chemical vapor deposition on planar m-plane sapphire substrates. Indium content in the semipolar material was compared with that in polar c-plane samples grown under the same conditions simultaneously side by side on the same holder. The investigated samples incorporated dual GaN/InGaN/GaN double heterostructures with 3nm wide wells. In order to improve optical quality, both polar and semipolar templates were grown using an in-situ epitaxial lateral overgrowth (ELO) technique. Indium incorporation efficiency was derived from the comparison of PL spectra measured on the semipolar and polar structures at the highest excitation density, which allowed us to minimize the effect of quantum confined Stark effect on the emission wavelength. Our data suggests increased indium content in the semipolar material by up to 3.0%, from 15% In in c- GaN to 18% In in (1122) GaN.

The realization of reliable single photon emitters operating at high temperature and located at predetermined positions still presents a major challenge for the development of solid-state systems for quantum light applications. We demonstrate single-photon emission from two-dimensional ordered arrays of GaN nanowires containing InGaN nanodisks. The structures were fabricated by molecular beam epitaxy on (0001) GaN-on-sapphire templates patterned with nanohole masks prepared by colloidal lithography. Low-temperature cathodoluminescence measurements reveal the spatial distribution of light emitted from a single nanowire heterostructure. The emission originating from the topmost part of the InGaN regions covers the blue-to-green spectral range and shows intense and narrow quantum dot-like photoluminescence lines. These lines exhibit an average linear polarization ratio of 92%. Photon correlation measurements show photon antibunching with a g(2)(0) values well below the 0.5 threshold for single photon emission. The antibunching rate increases linearly with the optical excitation power, extrapolating to the exciton decay rate of ~1 ns-1 at vanishing pump power. This value is comparable with the exciton lifetime measured by time-resolved photoluminescence. Fast and efficient single photon emitters with controlled spatial position and strong linear polarization are an important step towards high-speed on-chip quantum information management.

Nonpolar m-plane GaN layers were grown on patterned Si (112) substrates by metal-organic chemical vapor deposition (MOCVD). A two-step growth procedure involving a low-pressure (30 Torr) first step to ensure formation of the m-plane facet and a high-pressure step (200 Torr) for improvement of optical quality was employed. The layers grown in two steps show improvement of the optical quality: the near-bandedge photoluminescence (PL) intensity is about 3 times higher than that for the layers grown at low pressure, and deep emission is considerably weaker. However, emission intensity from m-GaN is still lower than that of polar and semipolar (1 100 ) reference samples grown under the same conditions. To shed light on this problem, spatial distribution of optical emission over the c+ and c− wings of the nonpolar GaN/Si was studied by spatially resolved cathodoluminescence and near-field scanning optical microscopy.

Diffusion lengths of photo-excited carriers along the c-direction were determined from photoluminescence (PL) measurements in p- and n-type GaN epitaxial layers grown on c-plane sapphire by metal-organic chemical vapor deposition. The investigated samples incorporate a 6 nm thick In0.15Ga0.85N active layer capped with either 500 nm p- GaN or 1300 nm n-GaN. The top GaN layers were etched in steps and PL from the InGaN active region and the underlying layers was monitored as a function of the top GaN thickness upon photogeneration near the surface region by above bandgap excitation. Taking into consideration the absorption in the active and underlying layers, the diffusion lengths at 295 K and at 15 K were measured to be about 92 ± 7 nm and 68 ± 7 nm for Mg-doped p-type GaN and 432 ± 30 nm and 316 ± 30 nm for unintentionally doped n-type GaN, respectively. Cross-sectional cathodoluminescence line-scan measurement was performed on a separate sample and the diffusion length in n-type GaN was measured to be 280 nm.

The optical quality of semipolar (1 101)GaN layers was explored by time- and polarization-resolved photoluminescence spectroscopy. High intensity bandedge emission was observed in +c-wing regions of the stripes as a result of better structural quality, while -c-wing regions were found to be of poorer optical quality due to basal plane and prismatic stacking faults (BSFs and PSFs) in addition to a high density of TDs. The high optical quality region formed on the +cwings was evidenced also from the much slower biexponential PL decays (0.22 ns and 1.70 ns) and an order of magnitude smaller amplitude ratio of the fast decay (nonradiative origin) to the slow decay component (radiative origin) compared to the -c-wing regions. In regard to defect-related emission, decay times for the BSF and PSF emission lines at 25 K (~ 0.80 ns and ~ 3.5 ns, respectively) were independent of the excitation density within the range employed (5 – 420 W/cm2), and much longer than that for the donor bound excitons (0.13 ns at 5 W/cm2 and 0.22 ns at 420 W/cm2). It was also found that the emission from BSFs had lower polarization degree (0.22) than that from donor bound excitons (0.35). The diminution of the polarization degree when photogenerated carriers recombine within the BSFs is another indication of the negative effects of stacking faults on the optical quality of the semipolar (1101)GaN. In addition, spatial distribution of defects in semipolar (1101)-oriented InGaN active region layers grown on stripe patterned Si substrates was investigated using near-field scanning optical microscopy. The optical quality of -c- wing regions was found to be worse compared to +c-wing regions due to the presence of higher density of stacking faults and threading dislocations. The emission from the +c-wings was very bright and relatively uniform across the sample, which is indicative of a homogeneous In distribution.

Semipolar (11macron01) GaN layers and GaN/InGaN LED structures were grown by metal-organic chemical vapor deposition on patterned (001) Si substrates. Optical properties of the semipolar samples were studied by steady-state and time-resolved photoluminescence (PL). Photon energies and intensities of emission lines from steady-state PL as well as carrier decay times from time-resolved PL were correlated with the distributions of extended defects studied by spatially resolved cathodoluminescence and nearfield scanning optical microscopy. Intensity of donor-bound exciton (DX) emission from both coalesced and non-coalesced semipolar layers is comparable to that of state-of-art c-plane GaN template. To gain insight into the contribution from near surface region and deeper portion of the layers to carrier dynamics in polar c-plane and semipolar (11macron01) GaN, time-resolved PL was measured with two different excitation wavelengths of 267 and 353 nm, which provide different excitation depths of about 50 nm and 100 nm, respectively. Time-resolve PL data indicate that the near-surface layer is relatively free from nonradiative centers (point and/or extended defects), while deeper region of the semipolar film (beyond of ~100 nm in depth) is more defective, giving rise to shorter decay times.

The temperature dependence of diffusion length and lifetime or diffusivity of the free exciton is measured in a
commercial ZnO-substrate and in an epitaxial ZnO quantum well using nm-spatially and ps-time resolved
cathodoluminescence (CL) spectroscopy. The characteristic temperature dependence of the exciton mobility is a
fingerprint of the underlying excitonic scattering processes. Since excitons are neutral particles scattering at ionized
impurities should be not effective. With decreasing temperature diffusion lengths and lifetimes give rise to a monotonous
increase of the excitonic mobility. Two different methods for determining the excitonic transport parameters will be
presented. On the one hand we are able to perform completely pulsed excitation experiments and on the other hand a
combination of cw- and pulsed excitation in two independent measurements are needed.

Semipolar (1-101) GaN layers were grown by metal-organic chemical vapor deposition on patterned (001) Si substrates.
The effects of reactor pressure and substrate temperature on optical properties of (1-101) GaN were studied by steadystate
and time-resolved photoluminescence. The optical measurements revealed that the optical quality of (1-101)-
oriented GaN is comparable to that of c-plane GaN film grown on sapphire. Slow decay time constants, representative of
the radiative recombination, for semipolar (1-101)GaN grown at 200 Torr are found to be very long (~1.8 ns), comparable
to those for the state-of-art c-plane GaN templates grown using in situ epitaxial lateral overgrowth through silicon nitride
nano-network. Defect distribution in the GaN stripes was studied by spatially resolved cathodeluminescence
measurements. The c+-wing regions of the GaN stripes were found to be dominated by a (D0,X) emission. Only a thin
slice of emission around 3.42 eV related to basal stacking faults was revealed in c--wing regions.

Non-polar (1-100 ) and semipolar (1-101)GaN layers were grown on (112) and (001) Si substrates, respectively, by metalorganic
chemical vapor deposition. In both cases, grooves aligned parallel to the <110> Si direction were formed by
anisotropic wet etching to expose vertical {111}Si facets for growth initiation. The effect of growth conditions (substrate
temperature, chamber pressure, ammonia and trimethylgallium flow rates) on the growth habits of GaN was studied. It
was found that low pressure and low ammonia flow rate are beneficial for m-facet formation, while high ammonia flow
rate promotes formation of (1-101) facets. Steady-state and time-resolved photoluminescence measurements revealed that
the optical quality of (1-101) oriented GaN is comparable to that of c-plane GaN film grown on sapphire. The nonpolar
(1-100 ) GaN shows only weak emission and fast non-radiative recombination rate. The poor optical quality of the mplane
GaN can be explained by carbon incorporation during the growth under low pressure. Although further
optimization of the growth conditions for better optical quality is required, preliminary results obtained for semipolar
(1-101) -oriented GaN are encouraging.

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